Radon (Rn-222) denotes a radioactive nuclide generated by the disintegration of uranium-series radium (Ra-226) and its half-life period is 3.824 days. Thoron (Rn-220) denotes a radioactive nuclide generated by the disintegration of thorium-series radium (Ra-224) and its half-life period is 55.6 seconds. These elements are both an inert gas and these origins are uranium (U-234) and thorium (Th-232) which occur in the earth crust, respectively.
Radon and thoron which have been generated in the earth crust are in the form of gas and therefore they leach out of the ground to the earth's surface or emerge on the ground together with groundwater. Then, if air ventilation is poor in highly airtight houses, insides of tunnels, underground shopping areas or the like, concentrations of radon and thoron reach a high level in some cases. The radiation exposure to which general public are subjected from the natural world in a year is said to be about 2.4 mSv. The exposure of a lung to the radiation caused by radon, thoron and their daughter radionuclides is said to account for approximately half of the exposure of 2.4 mSv. Accordingly, effects of the exposure due to these nuclides on a human body are concerned.
By the way, with respect to a method for measurement of airborne radon and thoron, an ionization chamber method, a scintillation cell method, an electrostatic collection type chamber method, a filter method and a still-standing-type measurement method have been presently put into practical use.
According to the ionization chamber method, a chamber with a capacity of 1 to 300 L is employed, and the method includes two, i.e., a method by measuring an ionized current and a method for measuring by using pulse. In order to increase the detection sensitivity of the methods, the capacity of the chamber should be enlarged and therefore the methods are unsuitable to perform high sensitive measurement in the field. Hence, the methods are often utilized as a measuring instrument for calibration. Consequently, in general, it is difficult to separately measure radon and thoron, using the methods.
According to the scintillation cell method, a device is employed which comprises a chamber having an inner wall to which fluorescent materials (ZnS:Ag) are applied and a photomultiplier tube connected optically to the chamber. Its measurement accuracy is proportional to a cell volume. When the cell volume is large, however, light from the fluorescent material becomes unable to reach the photomultiplier tube, thus putting restrictions on the cell volume. Further, unless radon concentration is comparatively high, it cannot be measured. Consequently, in general, radon and thoron are difficult to separately measure.
According to the electrostatic collection type chamber method, a spherical, hemispherical, or cylindrical chamber is employed. A mylar thin film is laid on a chamber bottom and a negative voltage is applied thereto, while a positive voltage is applied to a chamber wall. Polonium (Po-218), a daughter radionuclide of radon, is caught on the mylar film and detected. Radon and thoron can be separately measured but the measurement is strongly affected by humidity.
According to the filter method, the daughter radionuclides of radon and thoron which are in the atmosphere are collected directly on a filter to measure α rays emitted from the daughter radionuclides. In this method, radon is not directly collected and hence the equilibrium between radon and its daughter radionuclides must be estimated and the estimation could be an error factor. Further, according to the two-stage filter method, lowering of a measurement value due to the wall loss effect of the daughter radionuclides inside the chamber will pose a problem.
According to the still-standing (passive)-type method, resin such as polycarbonate or the like is laid as a detector inside a measurement container made of metal or the like. After having been atmospherically exposed for a preset period of time, the resin inside the detector is chemically etched to measure radon from α ray's solid state tracks developed there. The device used for this method is small-sized and hence a large number of the devices can be laid simultaneously at many measurement points and besides radon and thoron can be discriminated by using a filter or the like. Its detection sensitivity, however, is low and therefore the device needs to be atmospherically exposed typically for two moths or more. For the sake of reproducibly developing the solid state tracks, the chemical etching condition should be strictly controlled.
Other known prior art references are:
    Japanese unexamined patent publication No. H6-258443;    Japanese unexamined patent publication No. H6-258450;    Japanese unexamined patent publication No. H8-136660;    Japanese unexamined patent publication No. H8-136661;    Japanese unexamined patent publication No. H8-136662;    Japanese unexamined patent publication No. H8-136663; and    Japanese unexamined patent publication No. H8-201523.
As described above, various types of measurement methods have conventionally been put into practical use as the radon and thoron measurement methods. Either method, however, have had some sort of problems.
Therefore, in view of the problems set forth above, it is an object of the present invention to provide a novel method for measuring airborne radon and thoron in which a measurement device structure is small-sized and which is free of the influence from its measurement environment.